In the processing and packaging of semiconductor devices, wire bonding continues to be the primary method of providing electrical interconnection between two locations within a package (e.g., between a die pad of a semiconductor die and a lead of a leadframe). More specifically, using a wire bonder (also known as a wire bonding machine) wire loops are formed between respective locations to be electrically interconnected.
An exemplary conventional wire bonding sequence includes: (1) forming a free air ball on an end of a wire extending from a bonding tool; (2) forming a first bond on a die pad of a semiconductor die using the free air ball; (3) extending a length of wire in a desired shape between the die pad and a lead of a leadframe; (4) stitch bonding the wire to the lead of the leadframe; and (5) severing the wire. In forming the bonds between (a) the ends of the wire loop and (b) the bond site (e.g., a die pad, a lead, etc.) varying types of bonding energy may be used including, for example, ultrasonic energy, thermosonic energy, thermocompressive energy, amongst others.
As is known to those skilled in the art, these energy sources are not applied in a mutually exclusive way. For example, thermosonic energy typically involves the application of heat (e.g., from a heat block) and ultrasonic energy (e.g., from an ultrasonic transducer). When using ultrasonic energy in connection with wire bonding, there are generally two forms of ultrasonic output control: constant current control mode, where the current applied to the transducer is held constant (or is held to a predefined current profile) while the voltage may be varied; and constant voltage control mode, where the voltage applied to the transducer is held constant (or is held to a predefined voltage profile) while the current may be varied. In some applications, a constant power mode has also been used.
Many early wire bonder platforms used constant voltage mode in open loop control of current (i.e., the voltage applied is at a constant level regardless of the impedance variation of the system). Later wire bonder platforms adopted constant current control mode which allowed for more of a closed loop control. That is, the current may be fed back to a control board, whereby the voltage is adjusted to keep the desired current.
As is known to those skilled in the art, a benefit of constant current control mode is that it enables “portability” between one wire bonding system/machine and another. That is, the displacement of the transducer (and hence the capillary) is proportional to the current of the system. Therefore, in order to achieve similar bonding results (e.g., ball diameter, ball shear, etc.) for systems with different impedance values, supplying the same current to the transducer often yields acceptable results. One drawback of the constant current mode is that when a resonant frequency of the transducer is close a resonant frequency of the bonding components (e.g., a first bond die pad, a second bond lead of a leadframe), the impedance of the system may change significantly (e.g., the impedance may increase significantly). In a constant current control mode the system will attempt to adjust to this change in impedance by changing the voltage output (e.g., the voltage will significantly increase to account for a significant increase in impedance). For example, this may result in an increase to the overall energy put into the bond, which in turn may cause inconsistency in bonding (e.g., over squashed bonds, second bond short tail inconsistency, etc.).
In contrast, constant voltage control mode desirably limits the energy output to the bond when there is a resonance problem, and as such, the bonding results tend to be much more consistent. Unfortunately, a draw back of constant voltage control mode is a general lack of portability. Since the impedance of the systems are not the same (e.g., due to the mechanical differences of the transducer, coupling differences between the transducer and the mounting structure, mounting differences between the transducer and the capillary, etc.), the impedance from system to system may vary considerably (e.g., one system impedance may be 20 ohms, while another system impedance may be 50 ohms). Thus, when using the constant voltage control mode, the system with a lower impedance undesirably results in more energy being applied to the bonds than a higher impedance system.
As such, there are clear limitations in both the conventional constant current and constant voltage control modes. Thus, it would be desirable to provide improved methods of applying a constant voltage to an ultrasonic transducer of a wire bonding machine.